1. Field of the Invention
The present invention relates to an image readout apparatus for photoelectrically reading an image of an original document and, more particularly, to an image readout apparatus suitable for use in a facsimile apparatus for transmitting and receiving image data indicative of an image of an original document.
2. Related Background Art
The image readout apparatus provided in facsimile apparatus, digital copying apparatus, or the like is constituted so as to photoelectrically read an image of an original document in such a manner that the intensity of the reflected light from the original document exposed by a light source is detected by an image sensor such as a CCD line sensor or the like.
In general, shading correction is performed because the sensitivity of the original document readout sensor, the illumination of the illumination light source, and the like are not constant.
In the image processing to binarize an analog image signal on the basis of a proper level, hitherto, it has been proposed that the binarization level be changed in consideration of the shading in U.S. application Ser. No. 873,066 for example.
Namely, to cope with a fluctuation in light amount at a fluorescent lamp or lens (i.e., the peripheral light amount decreases), the slice level is determined in accordance with the fluctuation in the light amount without binarizing by a constant slice level.
In the actual facsimile apparatus or the like, just before an original document to be transmitted reaches the readout position, a reference white plate is scanned and the all white-signal is read out (hereinafter, this scanning operation is referred to as a prescan). A video signal waveshape corresponding to the light amount distribution at that time is stored. The slice level is determined using the stored video signal waveshape as a reference.
On the other hand, image transmission apparatuses having an original document feeding mechanism generally include two kinds of apparatuses: one is an apparatus wherein after a signal indicative of the communication link such as a signal representative of the non-voice terminal from a distant side receiver, an initial identification signal, or the like was detected, the feeding operation of an original document is started; the other is an apparatus wherein an original document is previously fed before the communication link is constructed.
According to the former apparatus, the transmission is started a predetermined period of time after the communication link is constructed so that a long period of time may be wasted.
In the case of the latter apparatus, although time can be effectively used, a problem occurs when the communication link is not constructed because the receiver is busy. When the communication link is not constructed, the reception side is ordinarily called again after a predetermined time has elapsed. However, if the original document moves during this time, the apparatus cannot accurately detect the edge position of the original document.
Therefore, as mentioned above, the reflected light from the reference white plate for the shading correction is sensed just before the readout operation of the original document is started. However, when the reception side is again called, the edge position of the original document cannot be detected as mentioned above. Therefore, it is unclear whether the white plate is read by the prescan operation or the original document itself is read out. Thus, there is the case where the original document is read by the prescan. In such a case, the shading correction cannot be accurately performed.
On the other hand, when the original document is deeply inserted beyond the readout position, the reference white plate is covered by the original document, so that the prescan cannot be executed. Such a situation in which the prescan cannot be executed, often occurs in the first page of the original document to be transmitted.
Therefore, if such a situation occurs, the binarization slice level is determined on the basis of the prescan data which was derived with regard to the last page in the preceding transmission of the image.
However, when the preceding image transmission was performed for a long period of time, the final prescan executed for this image transmission provides the all-white data when the fluorescent lamp (light source) is sufficiently warmed.
Moreover, when the time interval between the preceding image transmission and the subsequent image transmission is long, the fluorescent lamp is completely cooled.
Accordingly, if such a situation occurs in which the prescan cannot be performed when the fluorescent lamp is fully cooled, even if the slice level is determined using the data of the preceding prescan as reference data, the binarization cannot be properly performed.
In such a conventional image readout apparatus, since the light amount and light amount distribution of the light source (e.g., fluorescent lamp) largely change in dependence on the temperature of the pipe wall, there is a drawback such that the proper slice level cannot be set when the prescan cannot be performed.